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Full Discussion: question about popen in C
Top Forums Programming question about popen in C Post 302504575 by Corona688 on Tuesday 15th of March 2011 12:11:35 AM
Old 03-15-2011
You don't have to write your own popen(), stdio already has one.

Have you actually tried this code? It'll crash, because
Code:
return &fd[0];

That doesn't make a FILE pointer, just an integer pointer, since fd isn't a FILE. You convert a file number into a FILE * with a command like fdopen(fd, "r");

You also forgot to close the end of the pipe you're not using again.

This doesn't work either:
Code:
(*type == "r")

You don't compare strings that way.

Code:
if(strcmp(type, "r") == 0)
{
...
}

...and you have no way to wait for the process on close now, because you didn't store the PID anywhere. stdio popen() can do that and remember, but you can't just from a FILE *.

Honestly I think some review of the C language would do you good before you try to tackle multiprocessing.
 

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popen(3)						     Library Functions Manual							  popen(3)

NAME
popen - Initiates a pipe to a process LIBRARY
Standard C Library (libc.so, libc.a) SYNOPSIS
#include <stdio.h> FILE *popen ( const char *command, const char *type ); STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: popen(): XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about industry standards and associated tags. PARAMETERS
Points to a null-terminated string containing a shell command line. Points to a null-terminated string containing an I/O mode. DESCRIPTION
The popen() function creates a pipe between the calling program and a shell command to be executed. It returns a pointer to a FILE struc- ture for the stream. If the type parameter is the value r, the calling program can read from the standard output of the command by reading from the returned file stream. If the type parameter is the value w, the calling program can write to the standard input of the command by writing to the returned file stream. Because open files are shared, a type r command can be used as an input filter and a type w command as an output filter. NOTES
Programs using the popen() function to invoke an output filter should beware of possible deadlock caused by output data remaining in the program's buffer. This can be avoided by either using the setbuf() function to ensure that the output stream is unbuffered, or by using the fflush() function to ensure that all buffered data is flushed before calling the pclose() function. If the original processes and the process started with the popen() function concurrently read or write a common file, neither should use buffered I/O. If they do, the results are unpredictable. RETURN VALUES
Upon successful completion, the popen() function returns a pointer to the FILE structure for the opened stream. In case of error because files or processes could not be created, the popen() function returns a null pointer. RELATED INFORMATION
Functions: exec(2), fork(2), fclose(3), fopen(3), pclose(3), pipe(2), setbuf(3) Standards: standards(5) delim off popen(3)
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